Autism spectrum disorder (ASD) is an increasingly prevalent neurodevelopmental disorder affecting 1:110 children in the United States. Evidence supports large heritable contributions to the etiology of ASD, though environmental factors are likely to modify both the development and the course of ASD. Prenatal nutrition and nutrient-environment interactions have been understudied in relation to autism etiology and risk. As the first researchers to examine maternal nutrition in association with autism risk, we found that mothers of children with autism were significantly less likely to report having taken prenatal vitamins around conception than mothers of typically developing (TD) children. Additionally, their estimated total folic acid intake was lower. Here it is proposed that maternal nutrient statu may contribute to the multifactorial etiology of autism by modifying susceptibility to other environmental agents. Animal studies show that folic acid supplementation can protect the fetus from environmental toxins through DNA methylation mechanisms. Thus, the overall purpose of this R21 is to examine interactions between nutrient status and environmental exposures in relation to ASD and to assess DNA methylation as a candidate mechanism for the modification of susceptibility. The focus is on two classes of environmental exposures that have been linked to increased ASD risk: traffic-related air pollution (TRP) and pesticides. Preliminary findings suggest attenuation of these associations from maternal periconceptional prenatal vitamin supplement intake. More specifically an investigation of whether two measures of folate status, maternally-reported folic acid intake and newborn bloodspot folate, act as effect modifiers for TRP and pesticides, while considering timing and dose or level of each exposure, and adjusting for confounders will be explored. Expectations here are that higher levels of folic acid before and during early pregnancy and neonate folate will reduce the risk of ASD associated with TRP and pesticide exposures in a dose-dependent manner. Further, an examination of global DNA methylation as a mechanism by which folate status may modify susceptibility to the environmental exposure effects will be tested. It is hypothesized that higher levels of folate will counter the neurodevelomentally relevant hypomethylation effects of these environmental exposures by providing abundant access to methyl-groups, (reflected in higher global DNA methylation levels in newborn bloodspots), and reducing their associated ASD risk. The proposed study will not only elucidate whether folate is able to reduce the neurodevelopmental consequences of harmful environmental exposures, but will also help reveal critical time periods and mechanisms behind potential protective effects. If the hypotheses are supported, this will be one of the first studies to identify complex relationships among modifiable risk factors for autism to be followed up in a prospective study, where we can assess timing, dose of exposures, and precise mechanisms more thoroughly. As such, it is likely to have a measurable impact on the scientific community, potentially on nutritional and environmental policies, and could potentially identify strategies for ASD prevention.